Typical examples of the fluid flow battery include a redox flow battery (which may hereinafter be referred to as an RF battery). The RF battery performs charge and discharge by supplying a positive electrode electrolyte and a negative electrode electrolyte to a cell that includes a membrane and positive and negative electrodes that are disposed opposite each other, with the membrane interposed therebetween.
A water solution that contains metal ions (active materials) whose valence is changed by oxidation-reduction is typically used as an electrolyte. FIG. 4 illustrates a working principle of a vanadium-based RF battery 100 using a vanadium electrolyte that contains vanadium (V) ions as active materials for a positive electrode electrolyte and a negative electrode electrolyte.
The RF battery 100 includes a battery cell 100C divided into a positive electrode cell 102 and a negative electrode cell 103 by a membrane 101 that allows hydrogen ions to pass therethrough. The positive electrode cell 102 includes a positive electrode 104 therein, and a positive electrode electrolyte tank 106 configured to store the positive electrode electrolyte therein is connected to the positive electrode cell 102 through pipes 108 and 110. Similarly, the negative electrode cell 103 includes a negative electrode 105 therein, and a negative electrode electrolyte tank 107 configured to store the negative electrode electrolyte therein is connected to the negative electrode cell 103 through pipes 109 and 111. During charge and discharge, the electrolytes stored in the tanks 106 and 107 are circulated in the cells 102 and 103 by pumps 112 and 113. In FIG. 4, solid arrows in the battery cell 100C indicate a charge reaction and broken arrows in the battery cell 100C indicate a discharge reaction.
The battery cell 100C is normally stacked with other ones and used in the form of a structure called a cell stack 200, as illustrated in the lower part of FIG. 5. The cell stack 200 has a multilayer structure formed by stacking a plurality of battery cells 100C each including, as illustrated in the upper part of FIG. 5, the positive electrode 104, the membrane 101, and the negative electrode 105 that are stacked and sandwiched between cell frames 120. The electrodes 104 and 105 at both ends of the cell stack 200 in the stacking direction of the battery cells 100C are provided with respective current collector plates (not shown), instead of the cell frames 120. End plates 201 are disposed at both ends of the cell stack 200 in the stacking direction of the battery cells 100C. The end plates 201 in a pair are coupled together by coupling members 202, such as long bolts, therebetween and combined into a single structure.
The cell frames 120 each include a bipolar plate 121 made of plastic carbon (e.g., resin containing graphite) and a frame body 122 made of plastic and formed along the outer periphery of the bipolar plate 121. The frame body 122 is provided with liquid supply manifolds 131 and 132 and liquid supply slits 131s and 132s for supplying an electrolyte to each battery cell 100C, and liquid discharge manifolds 133 and 134 and liquid discharge slits 133s and 134s for discharging the electrolyte. The positive electrode electrolyte is supplied from the liquid supply manifold 131 through the liquid supply slit 131s formed on one side of the frame body 122 (i.e., on the front side of the drawing) to the positive electrode 104, and then discharged through the liquid discharge slit 133s to the liquid discharge manifold 133. Similarly, the negative electrode electrolyte is supplied from the liquid supply manifold 132 through the liquid supply slit 132s formed on the other side of the frame body 122 (i.e., on the back side of the drawing) to the negative electrode 105, and then discharged through the liquid discharge slit 134s to the liquid discharge manifold 134.
The cell stack 200 is formed by sequentially and repeatedly stacking one cell frame 120, the positive electrode 104, the membrane 101, the negative electrode 105, another cell frame 120, and so on. This allows the manifolds 131 to 134 to form an electrolyte flow path in the stacking direction of the battery cells 100C in the cell stack 200. The membrane 101 has substantially the same area as the cell frames 120 and is provided with through holes 101h at portions facing the respective manifolds 131 to 134 formed in the frame body 122 of each cell frame 120 (see PTLs 1 and 2). To prevent the electrolyte from leaking between the positive electrode cell 102 and the negative electrode cell 103 through the through holes 101h and also prevent the electrolyte from leaking through the through holes 101h to the outside of the cell stack 200, the through holes 101h in the membrane 101 are each generally provided with an O-ring (not shown) therearound.